1. Field of the Invention
This invention relates to photoluminescent indicators and in particular to indicators for indicating temperature and/or pressure conditions.
2. Description of the Prior Art
Photoluminescent thermometers are used in hostile environments, such as where magnetic fields and ultrasonic waves are present. One environment is found in the medical treatment of cancer by inducing local hypothermia. Other devices for use in such environments include liquid crystal, birefringent crystals, optical etalon sensors, fluid viscosity sensors and the like.
The determination of variable pressure conditions may also present problems in connection with requirements for such determinations in environments hostile to the conventional pressure sensing devices.
It is known that the decay curve of a photoluminescent material is both temperature and pressure sensitive. A number of devices have been developed for measuring the photoluminescent intensity decay so as to provide a corresponding indication of the temperature of the photoluminescent probe material.
One problem with using photoluminescent material to indicate either pressure or temperature conditions is that the decay curve of the photoluminescent material is not fully linear or exponential, but is a relatively complicated function that is difficult to predict. In the prior art structures, determination of the particular decay characteristics of a photoluminescent probe have been required to permit the use thereof for determination of temperature and pressure conditions.
Another problem inherent in such prior art structures is the problem of maintenance of accurate preselected levels of input light pulse intensity.
An extensive analysis of the use of optical sensors is set forth in Report No. NASA CR-159519. As discussed therein, Fabry-Perot temperature sensors may be utilized as remote fiber-optic transducers.
In "Phase Flurometry with Variable Duty Cycle `Electrical` Phosphoroscope," Journal of Physics E: Scientific Instruments, Vol. 9, 1013-1017 (1976), F. Gruneis, et al. disclosed the use of fast electrical switching means between a photomultiplier and a lock-in amplifier to allow the recording of a slow decaying component that is produced by a photoluminescent probe. The authors discussed the determination of the phosphorescent decay time by using an optical chopper with a variable speed. The authors suggest the use of a MOSFET transistor as a fast gate in the control apparatus. They point out that it is possible to do phosphorescent lifetime measurements by a phase shift technique.
In U.S. Pat. No. 4,075,493 to Wickersheim, the use of photoluminescent material is disclosed in connection with that technique of measurement wherein the phosphor material emits at least two optically isolatable wavelength ranges whose intensity ratio depends upon the object or environment temperature.
U.S. Pat. No. 4,179,972 to Saaski, discloses a temperature-sensing device having a temperature-responsive substance with a characteristic of changing in optical density with changes in temperature. The optical density changes are detected and monitored to indicate the temperature of the object.
In U.S. Pat. No. 4,140,393 to Cetas, a birefringent crystal thermometer is disclosed utilizing a beam splitter to generate two light beams. One light beam is passed through an optical fiber bundle for providing light to the photoluminescent probe. The second light beam generates a reference signal. Light returning from the probe is reflected by the beam splitter into an optical fiber which leads to a photodetector.
The concept of using photoluminescent thermometer probes in hostile environments such as microwave fields, is discussed in Science, Vol. 208, 193, 194 (1980). As taught therein, the luminescent response of the probe is delivered to a photomultiplier tube light detector through a fiber-optic bundle. However, in practice with this system, the temperature dependence of a given luminescent material must be experimentally determined for each sample of material. Further, this system does not include a feedback mechanism for maintaining the sensitivity of the system at a constant level over a range of temperatures.